The present invention relates to improved methods and apparatus for treating molten metal and more particularly to methods and apparatus for efficiently introducing reactive, volatile addition agents into such metal at increased rates.
In order to obtain desired metallurgical characteristics of a particular metallic product, it is frequently necessary to dissolve appropriate addition agents in the molten metal. However, many times such addition agents are highly volatile or reactive and thus, cannot readily be exposed to atmospheric oxygen prior to dissolution in the particular molten metal under treatment. Furthermore, many of such addition agents are of relatively low density and will tend to float on the surface of a molten metal such as iron rather than disperse substantially homogeneously therein. Thus, the mere charging of a quantity of such addition agents in either a lump or powdered form into a molten metal such as iron, presents extreme hazards to operating personnel due to the high reactivity of such agents and has also been unsatisfactory in providing a homogeneous dispersion and `recovery` of such addition agents in the molten metal. In addition, prior art techniques have been limited with respect to the quantities of agents that can be added to molten metal.
In order to avoid substantial losses of relatively expensive, addition agents, such as by reaction with atmospheric oxygen, it has been proposed to protect such addition agents during introduction into a molten bath. One such technique for protecting volatile addition agents is to encapsulate such agents as powder and/or solid wire in a tubular sheath and subsequently feed the composite or cored wire into a bath of the molten metal under treatment. In general, this technique for treating a molten metal bath has proven relatively effective in allowing the desired metallurgical characteristics of the bath under treatment to be obtained. However, in applications wherein the molten metal is flowing through a tundish or the like at a relatively rapid rate, on the order of 5-10 tons per min. in many steel mills, for example, one or more cored wires must be fed at extremely rapid rates in order to assure that sufficient quantities of addition agents are in fact introduced into the molten metal such that desired metallurgical characteristics can be realized. Under such circumstances, it has been found that merely increasing the wire feed speed in order to add sufficient quantities of addition agents results in a further serious problem, namely the fact that the wire is not fully melted by the molten metal before the wire contacts the bottom or other wall of the tundish or the like. This latter action causes the wire to bend, curl and re-emerge unmelted at the surface of the molten metal without providing the desired alloying effect. Similarly, in applications wherein a cored wire is supplied to a molten metal retained in an ingot mold or ladle, i.e., a stationary body of molten metal, it is none the less desired to augment as far as possible the rate of introducing such addition agents into the molten metal. It will be appreciated that the longer the molten metal is permitted to stand, temperature losses increase accordingly and reactive addition agents will "fade" or lose effectiveness, for example from oxidation beneath the melt surface. However, merely increasing the wire feed speed will result in problems similar to those previously described in connection with introducing addition agents into molten material flowing through a tundish.
Thus, present metallurgical techniques for treating molten metal with reactive and/or low density addition agents reflect a clear need for a process wherein such agents can be rapidly, and in a controlled manner without substantial loss, dissolved in a bath of molten metal.